CONICET | Buscador de Institutos y Recursos Humanos

The robber fly Mallophora ruficauda is one of the principal apiculture pests in the Pampas region of Argentina. As adults they prey on honey bees and other insects, while as larvae they are ectoparasitoids of third instar scarab larvae. Females of M. ruficauda lay eggs in tall grasses. After being dispersed by the wind, larvae drop to the ground, where they dig in search of their hosts. It is known that M. ruficauda larvae exhibit active host searching behaviour, however, it is unknown which instars are involved in this search. We carried out experiments in the laboratory to determine which larval stages are involved in host location. We report here that the second instar of M. ruficauda orientates specifically towards a source of Cyclocephala signaticollis odour, while first larval instar is indifferent to host cues. Furthermore, we have determined that second instar larvae are more motivated to initiate exploratory movements than larvae of the first stage. So far as we know, this is the first case among parasitoids, where the second instar is responsible for host location. Since in the Asilidae the genus Mallophora is the only one where parasitoids occur, we can speculate that a second host-seeking instar has evolved due to physiological constraints where a first larval instar that locates the microhabitat of the host is crucial.

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